Field of the Disclosure
The present application relates to an organic light emitting diode (OLED) display device. More particularly, the present application relates to an OLED display device and a driving method thereof adapted to prevent a brightness defect, such as a bright spot, using negatively shifted threshold voltage information of a driving transistor.
Discussion of the Related Art
As the information society spreads, the importance of flat panel display devices with features such as slimness, light weight, low power consumption and so on are being increased. The flat panel display devices include liquid crystal display (LCD) devices and OLED display devices which each include thin film transistors and have advantages of high definition, full color display, superior image quality and so on. The LCD devices and the OLED display devices are being applied to a variety of appliances such as television receivers, tablet computers, desk-top computer and so on. Particularly, the OLED display devices are being spotlighted as a next generation flat panel display device because of having high response speed, low power consumption, a self-luminous property and a wide viewing angle.
The OLED display device includes driving transistors (more specifically, driving thin film transistor) disposed pixels. The driving transistors must have different properties, such as threshold voltage Vth and mobility, due to a process deviation and so on.
FIG. 1 illustrates a negative shift phenomenon of a threshold voltage Vth of a driving transistor disposed on an OLED display device of the related art. FIG. 2 illustrates degrees of bright spot defects which are caused by negatively shifted degree of threshold voltages of driving transistors. FIG. 3 illustrates bright spots generated on the related art OLED display device which is displayed at a low gray level.
As shown in FIG. 1, the driving transistors used in the related art OLED display device can have negatively shifted threshold voltages NG from reference threshold voltages Ref due to a process deviation and foreign substances on driving transistor regions. The negatively shifted threshold voltage of the driving transistor can vary a driving current of an organic light emitting element of a pixel. Due to this, a bright defect can be generated in some pixels.
The bright defect can have one of brightness properties which are represented by first and second bright spot of FIG. 2 and depend on negatively shifted degrees of the threshold voltages.
Such bright defects can be displayed as bright spots, which have a higher brightness that those of adjacent spots thereto, and form yellow circles shown in FIG. 3, when the OLED display device is driven in a low gray level. In other words, stains and undesired patterns must be continuously displayed on the OLED device due to the bright defect.
In this manner, the process deviation and the foreign substance cause the bright defects by shifting the threshold voltage of the driving transistor of the related art OLED display device. Nevertheless, only one of changing and cleaning operations is performed to equipments, which are used in the fabrication of the OLED display device, without any method of removing the bright defect.